WASHINGTON (CNN) -- Police across the country should be on the lookout for what could be "dry runs" for a terrorist attack, the Transportation Security Administration advised after series of suspicious incidents occurred at U.S. airports.

Blocks of cheese were found in a bag with a coil of wire and tubes, the TSA said.

1 of 2 An unclassified advisory, sent July 20 from TSA to law enforcement agencies, raised the possibility that recent activity could be "pre-attack security probes."

CNN obtained the advisory from a government source.

The TSA downplayed the significance of the advisory in a statement released to the media following its leak.

The TSA said it was one of more than 90 bulletins sent to police in the past six months "with the intent to provide as much information as possible to our front line officers."

"There is no intelligence that indicates a specific or credible threat to the homeland," the TSA said.

The advisory details four incidents from the past 11 months in which screeners found unusual objects with items that could mimic bomb components in passengers' checked or carry-on bags. Watch how cheese was made to look like a bomb »

In one case last September, a couple in Baltimore, Maryland, checked a plastic bag with a block of processed cheese taped to another plastic bag containing a cell phone charger. Earlier this month in San Diego, California, a passenger checked a bag containing two ice packs covered in duct tape. The ice packs had clay in them instead of the normal blue gel.

Terrorists could be testing the system, or could be conducting repeated operations to desensitize security officials, the bulletin says.

"The unusual nature and increase in number of these improvised items raise concern, and TSA personnel should continue vigilance for groupings of ordinary items that look like IED (Improvised Explosive Device) components," the bulletin says.

Don't MissTSA: Fears about passenger taken off plane unfounded At least three of the four incidents involved U.S. citizens, the advisory says. While "initial investigations do not link them with criminal or terrorist organizations," the bulletin adds that "most passengers' explanations for carrying the suspicious items were questionable, and some investigations are still ongoing."

The bulletin also details a case from June in which a passenger in Milwaukee, Wisconsin, had a carry-on bag with items resembling IED components, such as a wire coil wrapped around a possible initiator, an electrical switch, batteries, three tubes and two blocks of cheese.

And in November, a passenger in Houston, Texas, checked luggage that contained a plastic bag with a 9-volt battery, wires, a block of brown clay-like minerals and pipes.

The bulletin is titled "Incidents at U.S. Airports May Suggest Possible Pre-Attack Probing." It is labeled "For Official Use Only."

"We constantly feed intelligence and training information to our officers and the law enforcement community and this is one example of such information sharing," the TSA said in its statement

sounds like a lot of discussion here can be boiled down to what has already been stated:

leave yourself extra time for the unexpected. I have flown dozens of times, pre-and post 9/11 with a rig, and my homemade li-ion batteries from 2002 that im still using are a bit on the sketchy-looking side.

i always make sure i have enough battery to power the rig, to turn the lights on and demo that it actually is a battery and a thing with lights.

usually batteries are :a. heavyb. cheap and replaceable (relative to other electronics)

so i try to check em

I took my gear bag out of my backpack for the first time last weekend, and didnt even get a hand-search, as they were 'less confused' when it came down the pipe

I had my best encounter yet leaving O'Hare for Raleigh last spring. The normal routine is that the run my bag through and then swab it for bomb residue (I'd hate to know if they could swab it for any other residue what kinda trouble I'd be in). This trip (my last one with SLAs) they run it through the scanner then the guy at the other side askes me if its mine and when I reply yes, he tells me to have a nice day. I ask him if they are going to swab my bag and he says, "No, we don't trust you anyway."